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[Previous: <a href="designer-creating-custom-widgets.html">Creating Custom Widgets for Qt Designer</a>]
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<h1 align="center">Creating Custom Widget Extensions<br /><small></small></h1>
<p>Once you have a custom widget plugin for <i>Qt Designer</i>, you can provide it with the expected behavior and functionality within <i>Qt Designer</i>'s workspace, using custom widget extensions.</p>
<ul><li><a href="#extension-types">Extension Types</a></li>
<li><a href="#creating-an-extension">Creating an Extension</a></li>
<li><a href="#exposing-an-extension-to-qt-designer">Exposing an Extension to Qt Designer</a></li>
<ul><li><a href="#creating-an-extension-factory">Creating an Extension Factory</a></li>
<li><a href="#accessing-qt-designer-s-extension-manager">Accessing Qt Designer's Extension Manager</a></li>
</ul>
<li><a href="#related-examples">Related Examples</a></li>
</ul>
<a name="extension-types"></a>
<h2>Extension Types</h2>
<p>There are several available types of extensions in <i>Qt Designer</i>. You can use all the extensions following the same pattern, only replacing the respective extension base class.</p>
<p><a href="qdesignertaskmenuextension.html">QDesignerTaskMenuExtension</a> is useful for custom widgets while <a href="qdesignercontainerextension.html">QDesignerContainerExtension</a> is necessary when implementing a custom multi-page container.</p>
<p><table width="100%" align="center" cellpadding="2" cellspacing="1" border="0">
<tr valign="top" class="odd"><td><img src="images/designer-manual-taskmenuextension.png" /></td><td><b><a href="qdesignertaskmenuextension.html">QDesignerTaskMenuExtension</a></b><p><a href="qdesignertaskmenuextension.html">QDesignerTaskMenuExtension</a> provides an extension that allows you to add custom menu entries to <i>Qt Designer</i>'s task menu.</p>
<p>See also the <a href="designer-taskmenuextension.html">Task Menu Extension</a> example.</p>
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<tr valign="top" class="even"><td><img src="images/designer-manual-containerextension.png" /></td><td><b><a href="qdesignercontainerextension.html">QDesignerContainerExtension</a></b><p><a href="qdesignercontainerextension.html">QDesignerContainerExtension</a> provides an extension that allows you to add (and delete) pages to a multi-page container plugin in <i>Qt Designer</i>.</p>
<p>See also the <a href="designer-containerextension.html">Container Extension</a> example.</p>
<p><b>Note:</b> It is not possible to add custom per-page properties to certain widgets (e.g&#x2e;, <a href="qtabwidget.html">QTabWidget</a>) due to the way they are implemented. This limitation will be addressed in future versions of Qt.</p>
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<p>The usage of <a href="qdesignermembersheetextension.html">QDesignerMemberSheetExtension</a> and <a href="qdesignerpropertysheetextension.html">QDesignerPropertySheetExtension</a> is more rare, but the classes enables you to manipulate the appearance of class members within <i>Qt Designer</i>'s workspace.</p>
<p><table width="100 %" align="center" cellpadding="2" cellspacing="1" border="0">
<tr valign="top" class="odd"><td><img src="images/designer-manual-membersheetextension.png" /></td><td><b><a href="qdesignermembersheetextension.html">QDesignerMemberSheetExtension</a></b><p>The <a href="qdesignermembersheetextension.html">QDesignerMemberSheetExtension</a> class allows you to manipulate a widget's member functions displayed when connecting signals and slots.</p>
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<tr valign="top" class="even"><td><img src="images/designer-manual-propertysheetextension.png" /></td><td><b><a href="qdesignerpropertysheetextension.html">QDesignerPropertySheetExtension</a>, <a href="qdesignerdynamicpropertysheetextension.html">QDesignerDynamicPropertySheetExtension</a></b><p>These extension classes allow you to manipulate a widget's properties displayed in <i>Qt Designer</i>'s property editor.</p>
</td></tr>
<tr valign="top" class="odd"><td></td><td><b>QDesignerScriptExtension</b><p>The QDesignerScriptExtension class allows you to define script snippets that are executed when a form is loaded. The extension is primarily intended to be used to set up the internal states of custom widgets.</p>
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<p><i>Qt Designer</i> uses the <a href="qdesignerpropertysheetextension.html">QDesignerPropertySheetExtension</a> and the <a href="qdesignermembersheetextension.html">QDesignerMemberSheetExtension</a> classes to feed its property and signal and slot editors. Whenever a widget is selected in its workspace, <i>Qt Designer</i> will query for the widget's property sheet extension, and whenever a connection between two widgets is requested, <i>Qt Designer</i> will query for the widgets' member sheet extensions.</p>
<p><b>Warning:</b> All widgets have default property and member sheets. But if you implement custom property sheet or member sheet extensions, these extensions will override the default sheets.</p>
<a name="creating-an-extension"></a>
<h2>Creating an Extension</h2>
<p>To create an extension you must inherit both <a href="qobject.html">QObject</a> and the appropriate base class, and reimplement its functions. Since we are implementing an interface, we must ensure that it's made known to the meta object system using the <a href="qobject.html#Q_INTERFACES">Q_INTERFACES</a>() macro in the extension class's definition. For example:</p>
<pre> class MyExtension: public QObject,
                    public QdesignerContainerExtension
 {
     Q_OBJECT
     Q_INTERFACE(QDesignerContainerExtension)

     ...
 }</pre>
<p>This enables <i>Qt Designer</i> to use the <a href="qobject.html#qobject_cast">qobject_cast</a>() function to query for supported interfaces using nothing but a <a href="qobject.html">QObject</a> pointer.</p>
<a name="exposing-an-extension-to-qt-designer"></a>
<h2>Exposing an Extension to Qt Designer</h2>
<p>In <i>Qt Designer</i> the extensions are not created until they are required. For that reason, when implementing extensions, you must subclass <a href="qextensionfactory.html">QExtensionFactory</a> to create a class that is able to make instances of your extensions. In addition you must register your factory with <i>Qt Designer</i>'s extension manager; the extension manager controls the construction of extensions as they are required.</p>
<p>When an extension is requested, <i>Qt Designer</i>'s extension manager will run through all its registered factories calling <a href="qextensionfactory.html#createExtension">QExtensionFactory::createExtension</a>() for each until it finds one that is able to create the requested extension for the selected widget. This factory will then make an instance of the extension.</p>
<p align="center"><img src="images/qtdesignerextensions.png" /></p><a name="creating-an-extension-factory"></a>
<h3>Creating an Extension Factory</h3>
<p>The <a href="qextensionfactory.html">QExtensionFactory</a> class provides a standard extension factory, but can also be used as an interface for custom extension factories. The purpose is to reimplement the <a href="qextensionfactory.html#createExtension">QExtensionFactory::createExtension</a>() function, making it able to create your extension, such as a <a href="designer-containerextension.html">MultiPageWidget</a> container extension.</p>
<p>You can either create a new <a href="qextensionfactory.html">QExtensionFactory</a> and reimplement the <a href="qextensionfactory.html#createExtension">QExtensionFactory::createExtension</a>() function:</p>
<pre>     QObject *ANewExtensionFactory::createExtension(QObject *object,
             const QString &amp;iid, QObject *parent) const
     {
         if (iid != Q_TYPEID(QDesignerContainerExtension))
             return 0;

         if (MyCustomWidget *widget = qobject_cast&lt;MyCustomWidget*&gt;
                 (object))
             return new MyContainerExtension(widget, parent);

         return 0;
     }</pre>
<p>or you can use an existing factory, expanding the <a href="qextensionfactory.html#createExtension">QExtensionFactory::createExtension</a>() function to enable the factory to create your custom extension as well:</p>
<pre>     QObject *AGeneralExtensionFactory::createExtension(QObject *object,
             const QString &amp;iid, QObject *parent) const
     {
         MyCustomWidget *widget = qobject_cast&lt;MyCustomWidget*&gt;(object);

         if (widget &amp;&amp; (iid == Q_TYPEID(QDesignerTaskMenuExtension))) {
              return new MyTaskMenuExtension(widget, parent);

         } else if (widget &amp;&amp; (iid == Q_TYPEID(QDesignerContainerExtension))) {
             return new MyContainerExtension(widget, parent);

         } else {
             return 0;
         }
     }</pre>
<a name="accessing-qt-designer-s-extension-manager"></a>
<h3>Accessing Qt Designer's Extension Manager</h3>
<p>When implementing a custom widget plugin, you must subclass the <a href="qdesignercustomwidgetinterface.html">QDesignerCustomWidgetInterface</a> to expose your plugin to Qt Designer. This is covered in more detail in the <a href="designer-creating-custom-widgets.html">Creating Custom Widgets for Qt Designer</a> section. The registration of an extension factory is typically made in the <a href="qdesignercustomwidgetinterface.html#initialize">QDesignerCustomWidgetInterface::initialize</a>() function:</p>
<pre> void MyPlugin::initialize(QDesignerFormEditorInterface *formEditor)
 {
     if (initialized)
         return;

     QExtensionManager *manager = formEditor-&gt;extensionManager();
     Q_ASSERT(manager != 0);

     manager-&gt;registerExtensions(new MyExtensionFactory(manager),
                                 Q_TYPEID(QDesignerTaskMenuExtension));

     initialized = true;
 }</pre>
<p>The <tt>formEditor</tt> parameter in the <a href="qdesignercustomwidgetinterface.html#initialize">QDesignerCustomWidgetInterface::initialize</a>() function is a pointer to <i>Qt Designer</i>'s current <a href="qdesignerformeditorinterface.html">QDesignerFormEditorInterface</a> object. You must use the <a href="qdesignerformeditorinterface.html#extensionManager">QDesignerFormEditorInterface::extensionManager</a>() function to retrieve an interface to <i>Qt Designer</i>'s extension manager. Then you use the <a href="qextensionmanager.html#registerExtensions">QExtensionManager::registerExtensions</a>() function to register your custom extension factory.</p>
<a name="related-examples"></a>
<h2>Related Examples</h2>
<p>Please see the <a href="designer-taskmenuextension.html">Task Menu Extension</a> and <a href="designer-containerextension.html">Container Extension</a> examples for more information about creating custom widget extensions in <i>Qt Designer</i>.</p>
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